Interest in solar cells has increased in recent years as an energy source with a low environmental impact.
Patent Document 1 and Patent Document 2 describe crystalline silicon solar cells using a crystalline silicon substrate. In the crystalline silicon solar cells described in Patent Document 1 and Patent Document 2, an uneven structure referred to as a textured structure is formed on the surface of the crystalline silicon substrate to increase the incident efficiency of the light on the crystalline silicon substrate.
Patent Document 2 describes the following method of forming a textured structure in a single-crystal silicon substrate. In the first step, the single-crystal silicon substrate is immersed in a 5 mass % sodium hydroxide aqueous solution for ten minutes at approximately 85° C. The first step removes the processing strain on the surface of the single-crystal silicon substrate. In the second step, anisotropic etching is performed on the surface of the single-crystal silicon using an aqueous solution containing approximately 2 mass % sodium hydroxide and isopropyl alcohol. Here, the concentration of sodium hydroxide is lower than in the first step. In the second step, a pyramidal textured structure is formed on the surface of the single-crystal silicon substrate. In other words, in Patent Document 2, a textured structure is formed by performing isotropic etching using an etching solution containing a relatively high concentration of etchant, and then performing anisotropic etching using an etching solution containing a relatively low concentration of etchant.